Slowing global warming no cause for complacency

Gerard Wynn

The latest forecasts from a foremost climate research institute that global warming has slowed present a new challenge to policymakers on how to inject urgency into the campaign to cut emissions of greenhouse gases.

Climate change is a growing problem. Each year in the past decade has been hotter than the 1981-2010 average, and extreme heat waves are becoming more frequent.

But the research indicates the rate of warming has slowed in the past decade and a half due to temporary natural factors.

Most scientists had dismissed the idea of a slowdown, citing the chaotic nature of weather which they say makes analysis of short-term trends meaningless, and an exceptional natural warming event (a strong El Nino) in 1998, which muddied the effect of rising human greenhouse gas emissions.

Warming has slowed, however, not only by comparison with 1998 but in the years since then. The new modelling from Britain's Hadley Centre, which forecasts global average temperatures to 2017, suggests the present decade may turn out to be no hotter than the last one.

That is something of a bombshell to the previous climate narrative of inexorable temperature rises decade by decade.

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"The earth is expected to maintain the record warmth that has been observed over the last decade," said Julia Slingo, Hadley Centre chief scientist, in a statement this week.

Natural influences are probably responsible for the slowdown, with candidates including a declining solar cycle since 2002 and the mixing of heat into the deep ocean, which has a vast storage capacity.

The basic physics of greenhouse gases still leaves no doubt that rising carbon emissions will push global temperatures up this century, and dangerously so.

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But the recent slower warming trend and the influence of natural variability present an extra headache for politicians trying to agree on climate targets for the next two decades, which already fall pitifully short of solving the problem.

Downgraded

The Hadley Centre trimmed its warming forecast over the next five years by 0.1 degree Celsius. It forecast global average temperatures would be 0.43 degrees higher than in the 1971-2000 period, compared with its previous forecast of 0.54 degrees.

The move followed a fine-tuning of the institute's models to better match ocean and atmospheric physics.

As well as the new forecast for the next five years, Stephen Belcher, head of the Hadley Centre, reported a "somewhat slower upward trend in the last 10 years".

He pointed to slower rises in sea surface temperatures.

Charts from the US National Oceanic and Atmospheric Administration (NOAA) illustrate how sea surface temperatures have lagged rises on land.

NOAA's "State of the Climate Report", published last July in the Bulletin of the American Meteorological Society, showed how the heat content of the top 2,300 feet of the ocean was rising more slowly than previously.

"Ocean heat content anomalies reached a new high in 2011, but they have been increasing more slowly since around 2003 than they did in the previous decade," the NOAA report found.

The Hadley Centre this week pointed to a cooling in the east Pacific Ocean, which it expected to continue.

"Were this cold phase to continue as forecast, this would act to moderate global warming in the next few years, as it has over the last decade," Slingo said.

Sun and sea

Emissions have risen steadily by 2 to 3 per cent a year over the past five decades, and the atmospheric concentration of carbon dioxide is rising in an almost perfectly straight line.

In a 2011 paper, Germany's Potsdam Institute for Climate Impact Research removed natural influences - the El Nino, changes in solar cycles and volcanic eruptions - to show that temperatures had otherwise risen steadily over the past three decades.

Volcanoes spew small particles called aerosols into the atmosphere which reflect sunlight and heat back into space.

The solar cycle may also help explain a recent fall in the Earth's surplus heat, US climate scientist James Hansen and co-authors reported in a 2011 paper, "Earth's energy imbalance and implications".

Greenhouse gases create a temporary, planetary energy imbalance by absorbing light at infra-red wavelengths, causing the atmosphere to radiate less heat into space and forcing the planet to warm up until it is once more in balance with the energy it gets from the sun.

But the solar cycle was in decline from around 2002-2009, cutting the energy reaching the Earth's surface."The longevity of the recent protracted solar minimum, at least two years longer than prior minima of the satellite era, makes that solar minimum potentially a potent force for cooling," Hansen and his co-authors said.

Another possible reason for slower warming involves an unexplained change in the rate at which oceans are soaking up the planet's excess heat.

The sea has a tremendous ability to absorb heat: the top two meters of ocean water contain about as much heat as the entire column of atmosphere above them, according to NOAA.

Whatever the explanation, the Hadley Centre is right to be transparent about the recent trend and outlook.

Its forecast of a slowing warming trend may appear to buy governments more time, but that is not at all the case. The trend results from temporary natural factors, and temperatures will rebound when their effects disappear.

The challenge for policymakers is to communicate that with the public and somehow pluck up courage to move ahead with climate negotiations and national efforts to cut emissions.